1. Field
The present disclosure generally relates to optical devices. More specifically, the present disclosure relates to an integrated multi-channel wavelength monitor which uses a diffractive echelle grating.
2. Related Art
High data rates are often achieved in optical telecommunication systems using wavelength-division-multiplexing (WDM) signals. WDM signals typically include light encoding data in multiple channels, each of which is at predefined channel or carrier wavelength. These carrier wavelengths are typically separated by 25 to 200 GHz within the 1528 to 1565 nm wavelength range.
An important component in WDM systems is a wavelength locker. This device stabilizes the output wavelength of a laser source (such as a laser diode), thereby ensuring that the carrier wavelengths conform to a standard (such as the International Telecommunication Union grid) and preventing channel crosstalk. This wavelength stabilization is often necessary when the channel spacing is decreased from 100 GHz to 50 GHz (or less) in order to compensate for factors that can shift laser-diode output wavelengths, such as: chip temperature variation, module-case temperature variation, bias-current changes and device aging.
As a consequence, wavelength lockers often include wavelength monitors that are used to provide feedback when locking laser sources. (In the discussion that follows, the combination of a wavelength monitor and a wavelength locker is sometimes referred to as an ‘optical-source monitor.’) Many commonly used wavelength monitors are based on thin-film filters and Fabry-Pérot etalons. However, these types of filter devices are usually costly, very bulky (e.g., several millimeter in size), and often require the corresponding discrete components (such as beam splitters and photo-detectors) to be placed on their own mounts, while the wavelength-monitoring devices are thermally coupled to a separate thermoelectric cooler (TEC). As shown in FIG. 1, which presents a block diagram of an existing front-facet wavelength monitor, the arrangement of the components can be very complicated, and usually requires very precise alignment between these discrete elements.
In addition, this type of wavelength monitor typically only works on a single wavelength at a time. In order to monitor multiple laser sources simultaneously, a set of wavelength monitors that work cooperatively may be needed, which can significantly increase the total package size and cost.
Hence, there is a need for an optical-source monitor that does not suffer from the above-described problems.